The present invention relates generally to implantable medical devices possessing a multiple cell power source. More particularly, this invention relates to an implantable cardioverter defibrillator that provides charge monitoring of a multiple cell power source so that if the charge of a cell is depleted, a patient warning signal is provided and the power source circuitry is reconfigured to rely only on cells with an acceptable remaining charge.
In the normal human heart, the sinus node, generally located near the junction of the superior vena cava and the right atrium, constitutes the primary natural pacemaker initiating rhythmic electrical excitation of the heart chambers. Disruption of the heart""s natural pacemaker and conduction system, as a result of aging or disease, can be successfully treated using implantable cardiac simulation devices, including pacemakers and implantable cardioverter defibrillator. A pacemaker generally delivers rhythmic electrical pulses to the heart to maintain a normal rhythm in patients having bradycardia, which is too slow of heart rate, or other conduction abnormalities. An implantable cardioverter defibrillator, commonly referred to as an xe2x80x9cICDxe2x80x9d, is capable of recognizing tachycardia or fibrillation and delivering electrical therapy to terminate such arrhythmias. ICD""s are often configured to also perform pacemaking functions as well.
Special difficulties arise in providing an adequate energy source for ICD""s, particularly those that are also intended to perform pacemaking functions. Cardioversion-defibrillator typically requires a few high-power electrical shocks to be generated relatively infrequently. For cardioversion, the shocks are typically at about two joules. For defibrillation, the shocks are typically at about twenty joules. Pacemaking functions, in contrast, may require that numerous relatively low-power electrical shocks be generated frequently. Pacing shocks are typically on the order of micro-joules. Energy is also required to monitor the heart for the purposes of detecting when cardioversion, defibrillation or pacing is required. Monitoring causes a continuous low-power current draw of about ten microamperes.
Also, energy may be required to reform whatever capacitor is used in connection with delivering defibrillation shocks. In this regard, aluminum electrolytic capacitors, which are commonly employed, typically must be charged to full voltage every couple of months to prevent degradation. Whether energy is actually required to perform capacitor reformation depends upon whether the patient receives relatively frequent defibrillation shocks. The ICD""s of patients that do not receive at least one defibrillation shock every month or two will require a periodic capacitor reformation cycle. The ICD""s of patients that do receive at least one defibrillation shock every month or two, however, do not typically require capacitor reformation cycles because reformation is achieved automatically during the generation of the defibrillation shocks.
To accommodate these various energy requirements, some ICD designs employ two power cells, a high-power cell and a low-power cell. Exemplary high-power cells include manganese dioxide cells and silver vanadium oxide cells. Exemplary low-power cells include carbon monofluoride cells and lithium iodine cells. In one possible example, the high-power cell provides energy for capacitor reformation and for cardioverter defibrillator functions and the low-power cell provides energy for the pacemaking and monitoring functions. In other possible examples, the high-power cell provides energy for capacitor reformation, cardioverter defibrillator functions and pacemaking functions and the low-power cell provides energy only for the monitoring functions.
The simplest ICD devices having separate high-power and low-power cells performed only monitoring, pacing and defibrillation functions and were configured to always draw energy for defibrillation from the high-power cell and to always draw energy for pacing and monitoring from the low-power cell, i.e. the power cells are non-switchable. However, the actual energy drawn from the low- and high-power sources varies considerably from patient to patient. For example, some patients require frequent pacing but little or no defibrillation whereas other patients require relatively frequent defibrillation but little or no pacing. Still others require neither pacing nor defibrillation but merely require continuous monitoring.
As a result of the wide variations in actual energy usage, circumstances can arise within ICD""s having non-switchable power sources wherein one power cell becomes quickly depleted thereby necessitating early replacement of the ICD even though the other power cell retains considerable energy and could otherwise continue to provide energy for the ICD. For example, circumstances can arise wherein the ICD must be replaced because the low-power cell has been depleted from frequent pacing or from a long period of continuous monitoring even though the high-power cell has abundant energy and could otherwise continue to power all ICD functions.
One design proposes a device that switches from the low-power cell to the high-power cell if the low-power cell becomes depleted, wherein energy for pacing and defibrillation is drawn from a silver vanadium oxide cell, and energy for monitoring is drawn from a lithium iodine cell. Thus, energy for monitoring is switched from the lithium iodine cell to the silver vanadium oxide cell if the lithium iodine cell becomes depleted.
Although this latter design represents an improvement over non-switchable systems, substantial room for further improvement remains. As an example, with that design, if the patient requires a considerable amount of pacing and a considerable amount of defibrillation therapy, the silver vanadium oxide cell will become quickly depleted, thereby necessitating early replacement of the device even though the lithium iodine cell retains considerable energy reserves.
In such circumstances, it would be preferable to switch the device, while the silver vanadium oxide cell still retains sufficient energy for defibrillation, to draw energy for the pacemaking functions from the lithium iodine cell to thereby slow the depletion of the silver vanadium oxide cell by more effectively using the remaining energy of the lithium iodine cell. However, this prior design does not provide for switching from the high-power cell to the low-power cell and thereby may result in a premature replacement of the ICD.
Moreover, even the manner by which this prior design operates to switch from the low-power cell to the high-power cell could be improved. In this regard, this prior design merely operates to determine whether the low-power cell has become completely depleted and, if so, switches completely and immediately to the high-power cell. Further improvement can be gained by gradually adjusting the relative amounts of energy drawn from the two power cells in an optimal manner.
In addition, in any battery-powered device, the device performance will eventually become compromised if one battery cell discharges below a functional level prior to device replacement. Particularly in the case of ICDs, such an event could be life-threatening to the patient. An elective or recommended replacement indicator provided by manufacturers of ICDs is used to indicate the recommended time for replacing the ICD. However, replacement indicators are not always accurate and have been found to underestimate the remaining battery life. While device replacement prior to the end of the battery life is crucial, improving the cost effectiveness of ICD therapy by extending the implant time is also a goal.
The cost effectiveness and safety of ICD therapy, therefore, can continue to be improved with improvements in battery technology. Hence, it would be desirable to provide a battery system that safely maximizes the implant time and that optimizes energy delivery from a power source to the ICD circuitry. Such capabilities would be desirable in an ICD regardless of the number of battery cells used or the battery chemistry implemented.
The present invention addresses these needs by providing an implantable cardioverter defibrillator (ICD) equipped with a multiple cell power source and switching circuitry for reconfiguring the power source when any cell charge precipitously decreases. The reconfigured power source selects the remaining cell or cells that retain an acceptable charge for powering monitoring and output functions of the ICD.
Furthermore the present invention provides that, upon detection of a reduced cell charge, a patient warning is issued to alert the patient that medical attention should be sought for determination of a safe device replacement time.
The foregoing and other features of the present invention are realized by providing an implantable, multichamber cardiac stimulation device equipped with pacing, cardioversion and defibrillation capabilities powered by a multiple cell power source. A preferred embodiment of the stimulation device includes a power source equipped with two batteries for powering the circuitry of the device, one battery having a higher resistance and greater energy density than the other battery; a control system for controlling the operation of the device; a set of leads that connects cardiac electrodes to the stimulation device for receiving cardiac signals and for delivering atrial and ventricular stimulation pulses; a set of sensing circuits comprised of sense amplifiers for sensing and amplifying the cardiac signals; a data acquisition system, such as an A/D converter, for sampling cardiac signals; and pulse generators for generating atrial and ventricular stimulation pulses.
The stimulation device further includes memory for storing operational parameters for the control system, such as stimulation parameter settings and timing intervals. The stimulation device also includes a telemetry circuit for communicating with an external programmer.
The power source is further equipped with switching circuitry such that battery cells may be selectively connected to the power source output for providing current to the sensing, output and control functions of the ICD. In a preferred embodiment, the lower energy density cell, preferably a lithium silver vanadium oxide cell, provides current flow to the circuitry of the stimulation device.
The higher energy density cell, preferably a lithium carbon monofluoride cell, provides current flow to the low density cell to maintain its charge. A current sensor provided between the high and low density cells detects if the current flow between the two cells deviates from a normal range. Upon detection of an unacceptable current flow, the control system opens or closes the appropriate switches such that the discharged cell is disconnected from the power supply output circuit The remaining cell continues to power all device functions.
The stimulation device is further provided with a patient warning system so that when a power cell is found by a control program to have a charge low enough to require reconfiguration of the power source circuitry, a low-battery warning signal is issued to the patient. The patient warning is preferably an audible buzzer or a twitch stimulation applied to excitable tissue surrounding the implanted device causing a sensation perceptible by the patient. The patient has been advised to seek medical attention upon perceiving the low-battery alarm such that an appropriate device replacement time may be scheduled.
The methods of the present invention thus improve the safety of the ICD by excluding a discharged power cell from the power supply circuitry so that remaining cells power all device functions adequately and by providing a patient warning of a low battery condition allowing the patient to seek medical attention well before device function becomes seriously impaired. The methods of the present invention may be advantageously applied using the most recent battery technology, which may use two or more cells of varying battery chemistries.